1. Field of the Invention
The present invention relates to a magnetoresistive effect device that is capable of recording information with a supply of bidirectional current, and a magnetic memory that includes the magnetoresistive effect device.
2. Related Art
A magnetoresistive random access memory (MRAM) that utilizes a tunneling magneto Resistive (TMR) effect characteristically stores data depending on the magnetization states of MTJ (Magnetic Tunnel Junction) devices. Generally, a MTJ device includes a magnetic layer called a storage layer, another magnetic layer called a reference layer, and a tunnel barrier layer interposed between the magnetic layers. Many techniques have been suggested for putting magnetoresistive random access memories into practice. For example, a yoke wiring structure has been suggested so as to reduce the write current. As for the structures of MTJ devices, a structure including a perpendicular magnetization film made of a GdFe alloy (see Ikeda, et al., “GMR and TMR Films Using GdFe Alloy with Perpendicular Magnetization”, Journal of the Magnetics Society of Japan, Vol. 24, No. 4-2, 2000, p.p. 563-566), for example), a stacked structure including a perpendicular magnetization film (see N. Nishimura, et al., “Magnetic Tunnel Junction Device with Perpendicular Magnetization Films for High-Density Magnetic Random Access Memory”, Journal of Applied Physics, Volume 91, Number 8, 15 Apr., 2002), and the like have been suggested.
In these suggested devices, the magnetic field write technique is utilized to reverse the magnetization direction of a magnetic layer with the use of a magnetic field generated from a current. A larger magnetic field can be generated from a larger amount of current, but the amount of current that can flow through the wirings is restricted due to miniaturization of devices. By employing a yoke structure in which the distance between the wirings and the magnetic layers is shortened or a magnetic field is concentrated in a certain region, the amount of current required for reversing the magnetization direction of a magnetic body can be reduced. However, a greater magnetic field is required for reversing the magnetization of a magnetic body due to miniaturization. Therefore, it is very difficult to restrict the amount of current to a small value and achieve miniaturization at the same time.
The reason that a greater magnetic field is required for reversing the magnetization of a magnetic body due to miniaturization is that a sufficient magnetic energy is required to overcome thermal agitation. To increase the magnetic energy, the magnetic anisotropy energy density and the volume of the magnetic body should be increased. However, since the volume becomes smaller due to miniaturization, the magnetic shape anisotropy energy or the magnetic crystalline anisotropy energy is normally used to increase the magnetic energy.
As described above, it is very difficult to restrict the amount of current to a small value and achieve miniaturization at the same time, since the reversal magnetic field is increased by the increase of the magnetic energy of the magnetic body. To counter this problem, a yoke structure of a completely closed magnetic circuit type has been suggested. This yoke structure includes a large perpendicular magnetization film that has a large magnetic crystalline anisotropy energy and maximum current field generation efficiency (see JP-A 2005-19464 (KOKAI), for example). In JP-A 2005-19464 (KOKAI), however, the yoke structure becomes large relative to the magnetic elements. As a result, the cell area becomes relatively large, and it is impossible to realize miniaturization, a smaller amount of current, and a reduction in cell area at the same time.
In recent years, magnetization reversals by spin-polarized current have been predicted in theory, and have also been confirmed through experiments. As a result, a MRAM that utilizes a spin-polarized current has been suggested (see J. C. Slonczewski, et al., “Current-Driven Excitation of Magnetic Multilayers”, Journal of Magnetism and Magnetic Materials, Volume 159, Number 1-2, L1-7, 1996, for example). In this MRAM, a magnetization reversal in a magnetic body by an action of spin-polarized electrons can be realized simply by applying a spin-polarized current to the magnetic body. If the volume of the magnetic body is small, the amount of spin-polarized electrons can be small. Accordingly, this technique is expected to achieve miniaturization and restrict the amount of current to a small value at the same time. Furthermore, this technique does not involve the magnetic field generated by a current. Accordingly, the yoke structure to increase the magnetic field is not necessary, and the cell area can be reduced.
In this magnetization reversing technique involving a spin-polarized current, however, the problem of thermal agitation becomes prominent with further miniaturization. As described above, to maintain sufficient thermal agitation resistance, it is necessary to increase the magnetic anisotropy energy density. In a conventional structure of an in-plane magnetization type, magnetic shape anisotropy is normally utilized. Since the magnetic anisotropy is secured with the use of its shape in this case, the current required for a magnetization reversal is sensitive to the shape, and the variation in the reversal current becomes larger with further miniaturization. Also, the aspect ratio of the MTJ cells needs to be at least 1.5 or more. As a result, the cell size becomes larger. Furthermore, the crystal axis diverges in a large area in the plane in a case where the magnetic layers of the magnetoresistive effect device are of the in-plane magnetization type and utilize magnetic crystalline anisotropy, instead of magnetic shape anisotropy, or in a case where a material having high magnetic crystalline anisotropy energy density such as a Co—Cr alloy material is used as in a hard disk medium. In such cases, the MR (Magneto-Resistive) ratio becomes lower, and incoherent precessional movement is induced. As a result, the amount of reversal current becomes larger.
To maintain information nonvolatility, a larger magnetic anisotropy energy than the thermal agitation energy should be supplied to the storage layer of each MTJ device. To secure a sufficient magnetic anisotropy energy, the use of a so-called perpendicular magnetization film having a magnetization easy axis in a direction substantially perpendicular to the film plane (such as the upper face) of the magnetization film has been suggested (see JP-A 2005-19464 (KOKAI), for example). Compared with a so-called in-plane magnetization film having a magnetization easy axis in a direction substantially parallel to the film plane, a perpendicular magnetization film requires a smaller amount of write current to reverse its magnetization. Accordingly, the use of perpendicular magnetization films is expected to be essential in large-capacity memory development (see JP-A 2007-142364 (KOKAI), for example).
Each MTJ device includes a storage layer, a reference layer, and a tunnel barrier layer interposed between the storage layer and the reference layer. The storage layer and the reference layer are made of magnetic materials, and generate a magnetic field outward. In a MTJ device having a storage layer and a reference layer of a perpendicular magnetization type, the leakage magnetic field from the reference layer is normally larger than the leakage magnetic field generated in a MTJ device of an in-plane magnetization type. Also, the storage layer having smaller coercive force than the reference layer is greatly affected by the leakage magnetic field from the reference layer. More specifically, due to the influence of the leakage magnetic field from the reference layer, the amount of reversal current required for writing is increased, and the thermal stability becomes poorer.
For a MTJ device of a perpendicular magnetization type, a SAF (Synthetic Anti-Ferromagnetic) structure, a dual-pin structure, and the like have been suggested as the measures to reduce the leakage magnetic field applied to the storage layer from the reference layer. In a SAF structure, a film thickness difference is caused, so as to reduce the leakage magnetic field applied to the storage layer. If the SAF structure is of an in-plane magnetization type, a film thickness ratio of approximately 1.2 should be sufficient for reducing the leakage magnetic field applied to the storage layer.
However, if the SAF structure is of a perpendicular magnetization type, a film thickness ratio of approximately 1.2 cannot sufficiently reduce the leakage magnetic field applied to the storage layer, as discovered by the inventors and will be described later.
In a dual-pin structure, the leakage magnetic field applied to the storage layer in the perpendicular direction is reduced, but the leakage magnetic field in the in-plane direction is increased. As a result, the MR ratio becomes lower, and the amount of reversal current required for writing becomes larger.
Although there have been some reports on the measures to reduce the leakage current in a MTJ structure of a perpendicular magnetization type as described above, no specific measures to reduce the leakage current applied to the storage layer have been suggested.